乳品加工手册chapter5

Chapter 5

Today the trend is towards progressively larger dairy units. The demand is for increased production without reduction in the quality of the finished product. Milk must be brought from farther away and this means that daily collection is generally out of the question. Nowadays collection usually takes place every other day, but the interval can often be three days and sometimes even four.Keeping the milk cool The milk should be chilled to below + 4°C immediately after milking and be kept at this temperature all the way to the dairy.If the cold chain is broken somewhere along the way, e.g. during trans-portation, the micro-organisms in the milk will start to multiply. This will result in the development of various metabolic products and enzymes.Subsequent chilling will arrest this development, but the damage has al-ready been done. The bacteria count is higher and the milk contains sub-stances that will affect the quality of the end product.Design of farm dairy premises The first steps in preserving the quality of milk must be taken at the https://www.wendangku.net/doc/ad8048056.html,king conditions must be as hygienic as possible; the milking system designed to avoid aeration, the cooling equipment correctly dimensioned.To meet the hygienic requirements, dairy farms have special rooms for refrigerated storage. Bulk cooling tanks are also becoming more common.These tanks, figure 5.2, with a capacity of 250 to 10 000 litres, are fitted with an agitator and cooling equipment to meet certain stipulations – for example that all the milk in the tank should be chilled to below +4°C within 2 hours of https://www.wendangku.net/doc/ad8048056.html,rger farms, producing large quantities of milk, often install separate coolers for chilling the milk before it arrives in the tank, figure 5.1. This saves mixing warm milk from the cow with the already chilled contents of the tank.The dairy room should also contain equipment for cleaning and disinfect-ing the utensils, pipe system and bulk cooling tank.Delivery to the dairy The raw milk arrives at the dairy in churns or in insulated road tankers, the latter being used only in combination with bulk cooling tanks at the farm.The requirements are the same for both methods – the milk must be kept well chilled and free from air and treated as gently as possible. For example,churns and tanks should be well filled in order to prevent the milk from sloshing around in the container.Churn collection Milk is transported in churns of various sizes, the most common being of 30or 50 litres capacity . The churns are taken from the farm to the roadside.

This should be done just before the arrival of the collecting lorry. The churns Fig. 5.2 Bulk cooling tank with agitator

and chilling unit

Fig. 5.1 The milk run in a closed system from cow to cooling tank

milk from heat and cold.

figure 5.3.

regions where there is no road to the dairy farm,

when water and/or electricity are not available on

the farm or when the milk quantities are too small to

justify investment in cooling facilities. The centres

can be organised in different ways and in accord-

ance with the prevailing situation. The farmers have

several alternatives. Uncooled milk in churns or

cooled milk in insulated tanks can be delivered at

certain road junctions, directly to tankers. Uncooled

milk can also be delivered in churns to centrally

placed cooling stations, figure 5.5. Another alterna-

tive is that neighbouring farmers deliver their un-

cooled milk in churns to a larger farm.

The churn-collecting lorry follows a carefully

planned schedule so that it always arrives at each

form of the lorry the churns should always be covered with a tarpaulin for protection against the sun and dust. The lorry returns to the dairy as soon as the churns have been collected from all the farms on its route.

is used by the dairy when calculating how much money the farmer should be paid.

Milk from diseased cows must not be supplied to the dairy together with milk from healthy animals. Milk from stock treated with antibiotics must be kept separate from other milk. Such milk cannot be used for products based on bacteria cultures, as the antibiotic strain will kill the bacteria. This applies to cultured milk products, cheese and butter, etc. Minute amounts of milk containing antibiotics can render enormous quantities of otherwise suitable milk unusable.Bulk collection

When milk is collected by tanker it must be possible to drive all the way to the farm dairy room. The loading hose from the tanker is connected to the outlet valve on the farm cooling tank. The tanker is usually fitted with a flow meter and pump so that the volume is automatically recorded. Otherwise the volume is measured by recording the level difference which, for the size of the tank in question, represents a certain volume. In many cases the tanker is equipped with an air eliminator.

Pumping is stopped as soon as the cooling tank has been emptied. This

delivers the milk to the dairy.Testing milk for quality

Milk from sick animals and milk which

contains antibiotics or sediment must not

be accepted by the dairy. Even traces of

antibiotics in milk can render it unsuitable

for the manufacture of products which are

acidified by the addition of bacteria cultures,

e.g. yoghurt and cheese.

Fig. 5.5 Farmers deliver uncooled milk in churns to centrally placed cooling stations

Fig. 5.6 Bulk collection at the farm

The following are the most common tests carried out on milk supplies.

Sediment tests

This applies only to churns. A sample is taken with a pipette from the bot-

tom of a churn and is then passed through a filter. A quality deduction is

made if visible impurities are retained by the filter.

Hygiene or Resazurin tests

The bacteria content of the milk is a measure of its hygienic quality. The

Resazurin Tests are used frequently. Resazurin is a blue dye which becomes

Two hygiene tests use this principle. One is a quick-screening test, which

The other test is a routine test and involves storage of the sample in a

°C for two hours.

Bacteria count

A simplified form of bacteria count can also be used to assess the bacteria

content. In this, the Leesment method, the bacteria are cultivated at 30 °C

for 72 hours in a 0.001 ml milk sample with a nutritive substrate. The bacte-

ria count is determined with a special screen.

Protein content

Many dairies pay the farmers according to the protein content of the milk.

This is analysed by means of instruments operating with infrared rays. Up to

300 analyses/hour can be performed.

Fat content

Various methods can be used to determine the butterfat content. The Ger-

ber test is the most widely used method for whole milk.

Freezing point

Many dairies check the freezing point of the milk to determine whether or

not it has been diluted with water. Milk of normal composition has a freezing Fig. 5.7 Milk from animals treated with

antibiotics must be kept separate from

other milk

point of -0.54 to -0.59 °

used for this check.

Milk reception

Dairies have special reception departments to

intake and compare it with the output.

ume or by weight.

Churn reception

lorry on a conveyor

tem is often designed so that the operator enters the producer identification

on a keyboard before weighing in all the churns from that producer, figure

5.9. The weights are then automatically totalled and recorded against the

identification. The identification for the next supplier is then entered by the

operator, and the process is repeated until all the milk has been weighed in.

The weighing equipment must be well maintained and checked every

day to ensure accuracy.

From weighing-in, the raw milk is pumped to storage tanks to await

processing.

The empty churns are conveyed to a cleaning station, where they are

washed with water and detergent to remove all traces of milk. In some

cases the clean churns continue to another station to be filled with feed-

stuff, which may be skimmilk, buttermilk or whey. Finally the churns contin-

ue to a loading dock to await return to the farm.

Tanker reception

Tankers arriving at the dairy drive straight into a reception hall, often large

enough to accommodate several vehicles.

The milk is measured either by volume or by weight.

Measuring by volume

This method uses a flowmeter. It registers the air in the milk as well as the

milk, so the results are not always reliable. It is important to prevent air from

entering with the milk. Measuring can be improved by fitting an air eliminator

before the flowmeter, figure 5.11.

Fig. 5.10 Measuring milk intake in a

tanker reception hall

The tanker outlet valve is connected to an air eliminator and from this the milk – free from air – is pumped through the flowmeter, which continuously indicates the total flow. When all the milk has been delivered, a card is placed in the meter for recording the total volume.

The pump is started by the control equipment which senses when the milk in the air eliminator has reached the preset level for preventing air from being sucked into the line. The pump is stopped as soon as the milk level drops below a certain level.

After measuring, the milk is pumped to a storage (silo) tank.

Measuring by weight

When the gross weight of the tanker has been recorded, the milk is delivered into the dairy. This may take place in line with a de-aerator but not a flowmeter. When empty, the tanker is weighed again and the tare weight is deducted from the previously recorded gross weight.

When the weighing-tank method is used, the milk is pumped from the tanker into a special tank with load cells built into the feet. The cells supply an electric signal that is always proportional to the weight of the tank. The strength of the signal increases with the weight of the tank as the milk en-ters the tank. The weight of the contents in the tank can be recorded when all the milk has been delivered. After this the milk is pumped to a silo tank. Tanker cleaning

Tankers are cleaned every day, as a rule at the end of a collection round. If the tanker makes several rounds a day, cleaning should take place after each round. Cleaning can be carried out by connecting the tanker to a cleaning system while in the reception area or by driving it to a special cleaning station.

Many dairies also clean the outside of their tankers every day so that they always look clean when they are on the road.

Chilling the incoming milk

Normally a temperature increase to slightly above + 4 °C is unavoidable during transportation. The milk is therefore usually cooled to below + 4 °C in a plate heat exchanger before being stored in a silo tank to await process-ing.

Raw milk storage

The untreated raw milk – whole milk – is stored in large vertical tanks – silo tanks – which have capacities from about 25 000 litres up to 150 000 litres. Normally, capacities range from 50 000 to 100 000 litres. Smaller silo tanks are often located indoors while the larger tanks are placed outdoors to reduce building costs. Outdoor silo tanks are of double-wall construction, with insulation between the walls. The inner tank is of stainless steel, pol-ished on the inside, and the outer wall is usually of welded sheet metal. Agitation in silo tanks

These large tanks must have some form of agitation arrangement to prevent cream separation by gravity. The agitation must be very smooth. Too violent agitation causes aeration of the milk and fat globule disintegration. This exposes the fat to attack from the lipase enzymes in the milk. Gentle agita-tion is therefore a basic rule in the treatment of milk. The tank in the illustra-tion 5.14 has a propeller agitator, often used with good results in silo tanks. In very high tanks it may be necessary to fit two agitators at different levels to obtain the required effect.

Outdoor silo tanks have a panel for ancillary equipment. The panels on the tanks all face inwards towards a covered central control station.

Tank temperature indication

The temperature in the tank is indicated on the tank control panel. Usually an ordinary thermometer is used, but it is becoming more common to use an electric transmitter, which transmits signals to a central monitoring sta-tion.

Level indication

There are various methods available for measuring the milk level in a tank. The pneumatic level indicator measures the static pressure represented by the head of liquid in the tank. The greater the pressure, the higher the level in the tank. The indicator transmits readings to an instrument.

Fig. 5.14 Silo tank with propeller agitator

Low-level protection

All agitation of milk must be gentle. The agitator must therefore not be start-

Fig. 5.15 Silo tank with alcove for manhole,

indicators, etc.

1 Agitator

2 Manhole

3 Temperature indicator

4 Low-level electrode

5 Pneumatic level indicator

6 High-level electrode